Supporting Information for
Decoupling optical and electronic optimization of organic solar cells using
high-performance temperature-stable TiO2/Ag/TiO2 electrodes
Kwang-Dae Kim,1 Thomas Pfadler,1 Eugen Zimmermann,1 Yuyi Feng,1 James. A.
Dorman2, Jonas Weickert,1,a), and Lukas Schmidt-Mende1,b)
1Department
2Chemical
of Physics, University of Konstanz, POB 680, 78467 Constance, Germany.
Engineering Building, Louisiana State University, LA 70803, Baton Rouge, USA
a)
Electronic mail: jonas.weickert@uni-konstanz.de
b)
Electronic mail: lukas.schmidt-mende@uni-konstanz.de
FIG. S1. Optical electrical field profile into OPV system. Simulated result of the optical electrical field profile for
OPVs structured as Ag (150 nm)/PEDOT (8 nm)/PTB7-PC71BM (95 nm) on (a) TiO2 (20 nm)/ITO (170 nm) and
(b) TiO2 (20 nm)/Ag (12 nm)/TiO2 (28 nm) shown in Figure 1b depending on the wavelength of incident light (450800 nm).
1
Figure S2 shows the sheet resistance, transmittance, and figure of merit (FOM) value of T/ITO and
TAT with different thickness of bottom-TiO2 layer. The sheet resistances were gradually increased in the
TAT electrode with increasing Tbottom-TiO2 from 5 to 55 nm. These values show a lower value in comparison
to that of ITO. The transmittance at a wavelength of 550 nm was decreased as a function of the increase
in the Tbottom-TiO2 (Figure S2a). However, a different behavior was observed at a wavelength of 676 nm,
which corresponds to the absorption peak of PTB7 as photoactive material (Figure S2b). By increasing
Tbottom-TiO2 from 5 to 28 nm, the transmittance was gradually increased, and the highest value was observed
at 28 nm. By increasing Tbottom-TiO2 from 40 to 55 nm, it was gradually decreased.
Since a TCE is a compromise material due to confliction between transmittance and conductivity, a
FOM has been used to compare TCE.1 The FOM value of the T/ITO and TAT multilayer electrodes were
calculated from the sheet resistance and optical transmittance (ΦTC=T10/Rsh).1, 2 In our TAT electrode with
different thickness of bottom-TiO2 layer, the FOM values follow a tendency of the optical transmittances.
When FOMs for two different electrodes of T/ITO and TAT: 20/12/28 nm multilayer, which are used for
fabrication of OPV in Figure 1b, are compared, these values of T/ITO are higher than that of TAT
multilayer. These results can indicate higher possible photovoltaic performance in the ITO system,
however; the performance of OPV shown in the Figure 1b is in discord with FOM results. It should be
noted that not only optoelectronic properties of electrode but also absorption behavior, i.e., the coherent
electric field intensity (|E|2) distribution inside the active layer of the device should be considered to
maximize device performance. Therefore, we can strongly suggest that the tuneability of optical cavity is
unconditionally required for the high-performance state of the art OPVs.
FIG. S2. Optical and electrical properties of T/ITO and TAT electrodes. Sheet resistance (Ω/square), transmittance
(%) and figure of merit (10-3Ω-1) for T/ITO (in the box with diagonal lines) and TiO2/Ag/TiO2 (TAT) with different
thicknesses of bottom-TiO2 layer at (a) 550 and (b) 676 nm, which is corresponded to maximum peck of PTB7 as
photoactive material, wavelengths of light. The thicknesses of Ag and top-TiO2 layers are 12 and 20 nm,
respectively. These values were measured after annealing at 400 oC for 30 min under vacuum condition of 3x10-3
Torr.
2
Table SΙ. Performance of devices for different electrode. Averaged performance of device for T/ITO and TAT with
different thickness of bottom-TiO2 (5, 10, 28, 40 and 55 nm). It was measured under 100 mW/cm2AM 1.5G solar
illumination.
PCE (%)
Device
FF (%)
Voc (V)
2
JSC (mA/cm )
Average / Best
ITO
62.5±1.6
0.74±0.01
15.43±0.35
7.13±0.25 / 7.50
5 nm
62.5±1.1
0.74±0.01
17.08±0.22
7.86±0.20 / 8.09
10 nm
62.5±1.5
0.74±0.01
16.89±0.23
7.81±0.19 / 8.05
28 nm
63.5±1.8
0.75±0.01
17.78±0.31
8.45±0.19 / 8.73
40 nm
60.2±2.5
0.74±0.01
16.30±0.28
7.26±0.46 / 7.82
55 nm
52.1±4.1
0.72±0.04
15.49±0.80
5.84±0.83 / 7.01
Thickness of
bottom-TiO2 layer
FF; fill factor, Voc; open circuit voltage, Jsc; short circuit current, PCE; power conversion efficiency.
FIG. S3. (a) EQE and (b) total absorption spectra of devices for T/ITO and TAT (ITO-free) electrodes with different
thickness of bottom-TiO2 (5, 10, 28, 40 and 55 nm).
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FIG. S4. Relative EQE and total absorption of devices with different thickness of bottom-TiO2; (a) 5, (b) 10, (c)
28, (d) 40 and (e) 55 nm. These values were calculated as (TAT-T/ITO)/TAT.
To verify the surface morphology change depending on variation of the bottom TiO2 layer
thickness with annealing process, the T/ITO and TAT electrodes were characterized by AFM. Figure S4
shows the topology of electrodes, the root mean square (RMS) roughness (Rrms) values form AFM
measurement were plotted in Figure S6. A slight increase of the Rrms value was observed in all electrodes
after annealing process. It can be resulted from an increase of the grain size by crystallization of TiO2
during the annealing process. In the AFM images of T/ITO electrode, the ITO surface feature with higher
Rrms value was obtained when compared with that of TAT electrodes. In contrast, the smooth surface
was observed in the TAT electrodes. Additionally, no significant change depending on the bottom TiO 2
layer thickness was observed, although the Rrms value is slightly increasing as a function of the bottom
TiO2 layer thickness from 5 to 55nm.
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FIG. S5. AFM images for (a, g) T/ITO and (b-f and h-l) TAT with different thickness of bottom-TiO2 (5, 10, 28,
40 and 55 nm) (a-f) with and (g-l) without annealing at 400 oC for 30 min.
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FIG. S6. Root mean square (RMS) roughness (Rrms) values from AFM measurement (in Figure S3) for T/ITO and
TAT with different thickness of bottom-TiO2 (5, 10, 28, 40 and 55 nm) with and without annealing at 400 oC for 30
min.
The transmittance and sheet resistance of TAT electrodes with different thickness of Ag were
characterized to determine the optimum thickness of Ag (Figure S7, Table SΙΙ in below). A blue shift at
wavelength of maximum intensity was observed in the transmittance spectra with increasing thickness of
Ag. The transmittance at a wavelength of 550 nm was increased as a function of the increase in the Ag
thickness. A similar transmittance for electrodes with 10 and 12 nm of Ag thickness was observed at a
wavelength of 676 nm, which corresponds to the absorption peak of PTB7 as photoactive material. The
sheet resistances were gradually decreased in the TAT electrode with increasing thickness of 8.5 to 12
nm. The TAT electrode with 12 nm thickness of Ag shows the lowest resistance which is 6.93 Ω/ square.
We calculated the FOM at 550 nm and 676 nm from transmittance and sheet resistance results to evaluate
each electrodes. For the thickness of Ag is 12 nm, the highest FOM values of 24.88 and 23.92 were
observed at both 550 nm and 676 nm wavelengths, respectively. Hence, the 12nm thickness of Ag was
used in this study.
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FIG. S7. Transmittance spectra for TAT (ITO-free) electrodes of bottom-TiO2 (20 nm)/Ag (x nm)/top-TiO2 (30
nm) with different thickness of Ag (8.5, 10, and 12 nm).
Table SΙΙ. Transmittance (T), sheet resistance (Rsh) and figure of merit (ϕ) of electrode for different thickness of
Ag. The performance of electrode of bottom-TiO2 (20 nm)/Ag (x nm)/top-TiO2 (30 nm) with different thickness of
Ag (8.5, 10, and 12 nm) was summarized.
at 550nm
at 676nm
TiO2
Ag
TiO2
Rsh
(nm)
(nm)
(nm)
(Ω/square)
T (%)
ϕ (=T10/Rsh)
T (%)
ϕ (=T10/Rsh)
20
8.5
30
14.34
75.26
4.06
79.55
7.08
20
10
30
8.83
80.97
13.72
84.48
20.97
20
12
30
6.93
83.88
24.88
83.55
23.92
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METHODS
Preparation of ITO-free electrode. The TAT multilayer was sequentially deposited onto glass substrate
(PG&O) by dc sputtering of a TiO2 target (99.99% purity) and Ag target (99.99% purity) at room
temperature in a high vacuum system with a base pressure of 3x10-7 Torr. TiO2 and Ag were deposited
using dc power of 100 W. The thicknesses of the TiO2 layer and Ag were calibrated using spectroscopic
ellipsometry and atomic force microscope (AFM, Bruker, Multimode 6), respectively.
OPV device fabrication. To fabricate the inverted OPV devices, layers of a donor-acceptor blend,
PEDOT:PSS, and Ag top-electrode were sequentially deposited onto TiO2-coated ITO glass and TATcoated glass (glass size: 1.4 × 1.4 cm2) which were annealed at 400 °C for 30 min under vacuum condition
(5x10-3 Torr). For the preparation of the active layer, PTB7 and PC71BM (8:12 mg) were dissolved in
chlorobenzene/1,8-diiodooxtane (CB/DIO; 0.97/0.03 ml, Aldrich). Both donor and acceptor polymers are
commercially available. The solution was stirred at 45 °C overnight. The active layer was deposited onto
the TiO2-coated ITO glass (Lumtec) and TiO2/Ag/TiO2 coated glass substrate using spin-coating at 1000
rpm for 40 sec under N2 atmosphere. For the preparation of the PEDOT:PSS (Clevios P VP AI 4083,
Heraeus) solution, PEDOT:PSS and IPA were mixed in a ratio of 1:10. This solution was spin-coated onto
the active layer at 5000 rpm for 60 sec under N2 condition. The Ag metal (150 nm) top electrode was
thermally evaporated onto the PEDOT:PSS layer at an evaporation rate of 1 Å s -1 at a pressure of 5x10-6
Torr.
Characterization. The sheet resistance (Rsh) values were measured using a 4-point probe system with a
Keithley 2400 Source Meter. J-V curves and EQE of the OPV devices were taken using a Keithley 2400
Source Meter controlled through a self-written LabView program. For J-V measurement, devices were
illuminated via a LOT-Oriel LS0106 solar simulator under AM 1.5 simulated illumination of 100 mW/cm2.
A shadow metal mask with an active cell area of 0.125 cm2 was used during measurement, which covered
back and sides of the solar cell samples to allow exact determination of the active area. The intensity of
simulated sunlight was measured using a certified Si reference photovoltaic cell (Fraunhofer Institute). To
control the illumination wavelength for the EQE measurements, light from a 150 W Xe lamp was passed
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through a LOT-Oriel Omni 150 monochromator. The absorption spectra were measured by a Cary 5000
series UV-Vis-NIR spectrometer of Agilent Technologies.
Simulation. Simulations of coherent electric-field distributions, local absorption, and charge generation
within our device configuration were done with an adjusted version of the Transfer Matrix Optical
Modeling
Script
by
Burkhard
et
al.3
(downloaded
from
http://web.stanford.edu/group/mcgehee/transfermatrix/). This method calculates the reflection and
transmission at each interface and the interference between the incident electric field and the fields
reflected from the Ag back electrode as well as attenuation in each layer. Field patterns have been
calculated for wavelengths between 350 and 800 nm, and the lattice point distance for calculation of the
electric field was taken as 1 nm. Dispersion relations of refractive indices and extinction coefficients of
ITO, PEDOT:PSS, P3HT:PC61BM blend, and silver have been taken from the database attached to the
simulation program package. Values of anatase TiO2 by E. F. Schubert4 were downloaded from
http://homepages.rpi.edu/~schubert/, and values of the PTB7:PC71BM blend have been extracted from
Hedley et. al.5 Figure S8 shows dispersion relations of refractive indices and extinction coefficients of
main materials used in simulation.
FIG. S8. (a) Refractive indices; n and (b) extinction coefficients; k of used materials PTB7:PC 71BM,
P3HT:PC61BM, ITO, Ag, and TiO2. Values for ITO, P3HT:PC61BM and Ag have been taken from the database of
the simulation software, and values of anatase TiO2, and PTB7:PC71BM were taken from literature.4, 5
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Reference
1
D. S. Ginley, H. Hosono, D. C. Paine, Handbook of transparent conductors, (Springer, New York, 2010)
Cp. xiii, p. 533.
2
G. J. Haacke, Appl. Phys. 47, 4086 (1976).
3
G. F. Burkhard, E. T. Hoke, M. D. McGehee, Adv. Mater. 22, 3293 (2010).
4
E.
F.
Schubert,
Materials
Refractive
index
and
extinction
coefficient,
http://homepages.rpi.edu/~schubert/. accessed: Oct, (2014).
5
G. J. Hedley, A. J. Ward, A. Alekseev, C. T. Howells, E. R. Martins, L. A. Serrano, G. Cooke, A.
Ruseckas, I. D. W. Samuel, Nat. Commun., 4, 2867 (2013).
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